Far beneath the bustling streets of Manhattan, and out of sight for the majority of New York City’s 8 million residents, the largest transportation project in the country is churning away day and night hollowing out granite to create six miles of new tunnels.

It’s the biggest addition to New York’s public transportation system since the opening of Penn Station in 1910; a push to connect the Long Island Rail Road to Grand Central Terminal. When completed, 32,000 feet of new tunnels will have been excavated.

Thus far, all digging has been done by a pair of 22-foot long hard rock boring machines; to date, nearly 22,200 feet of tunnel has been cleared. Outside of Manhattan, two soft-ground tunnel boring machines will begin excavating 10,000 linear feet of tunnels in Queens to connect the 63rd street Tunnel to the existing LIRR tracks in Sunnyside. If you’ve never visited the Big Apple, take it from this New Yorker: such connections will change the lives of hundreds of thousand of commuters each week.

Construction work began in 2006, but is so complicated and rigorous that the end date of the construction project has been pushed back multiple times. The new tunnels are now expected to open in 2019.